Within the metallurgical arts, many types of corrosion may exist. Galvanic corrosion is a particular form of corrosion characterized by an electrochemical process in which one metal corrodes preferentially to another metal. For galvanic corrosion to exist, both metals must be in electrical contact and immersed in an electrolyte. Such a configuration is often referred to as a “galvanic couple” and results when each metal has a different electrode potential. Various metals are often ranked according to their electrode potential in the “Anodic Index,” with magnesium having a generally higher potential than steel, steel having a generally higher potential than copper, and gold having the lowest potential.
In a galvanic couple, the electrolyte provides a means for transport of metallic ions and the electrical contact ensures migration of electrons from the more anodic metal to the less anodic metal (i.e., the more cathodic metal). This leads to the anodic metal corroding more quickly than it otherwise would, while the corrosion of the cathodic metal is retarded even to the point of stopping.
Alternative modes of corrosion may include crevice corrosion, and/or pitting corrosion. In crevice and pitting corrosion, for example, a pseudo-galvanic couple is achieved between two regions of a single piece of metal. For example, the polarization of the metal within the crevice may be altered by the presence of a stagnant electrolyte. This altered potential may, for example, serve as the anode and corrode preferentially to the remainder of the metal, which may be more cathodic.